Chemical-mechanical planarization slurries and powders and methods for using same

ABSTRACT

Chemical-mechanical planarization slurries and methods for using the slurries wherein the slurry includes abrasive particles. The abrasive particles have a small particle size, narrow size distribution and a spherical morphology and the particles are substantially unagglomerated.

This application claims priority from U.S. Provisional Application Nos.60/038,263 and 60/039,450, both filed on Feb. 24, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to abrasive powders and slurriescontaining the powders, as well as chemical mechanical planarizationprocesses utilizing the slurries. In particular, the present inventionis directed to such powders and slurries wherein the abrasive powder hasa small average particle size, controlled particle size distribution,spherical morphology and is substantially unagglomerated.

2. Description of Related Art

Slurries consisting of abrasive and/or chemically reactive particles ina liquid medium are used for a variety of polishing and planarizingapplications. Some applications include polishing of technical glass,magnetic memory disks, native silicon wafers and stainless steel used inmedical devices. Chemical-mechanical planarization (CMP) is a method toflatten and smooth a workpiece to a very high degree of uniformity. CMPis used in a variety of applications including polishing of glassproducts such as flat panel display glass faceplates and planarizationof wafer-based devices during semiconductor manufacture. In particular,the semiconductor industry utilizes CMP to planarize dielectric, metalfilm as well as patterned metal layers in the various stages ofintegrated circuit manufacture.

CMP consists of moving a nonplanarized, unpolished surface against apolishing pad at several PSI of pressure with a CMP slurry disposedbetween the pad and the surface being treated. This is typicallyaccomplished by coating the pad with a slurry and spinning the padagainst the substrate at relatively low speeds The CMP slurry includesat least one of two components: an abrasive powder for mechanical actionand solution reactants for chemical action. The solution reactants aretypically simple complexing agents or oxidizers, depending on thematerials to be polished, and acids or bases to tailor the pH. Forpolishing metal layers, the slurry will predominately polish the metalthrough the action of the solution reactants. The abrasive powder isprimarily responsible for the mechanical abrasion at the surface, butcan also contribute to the chemical action near the surface.

As the powder abrades the surface, protrusions and other irregularitiesare removed. The chemical species in the slurry can perform differentfunctions, such as aiding in the dissolution of the mechanically removedmaterial by dissolving it into solution or oxidizing the surface layersto form a protective oxide layer. The particular solution chemistry isprimarily dependent upon the material being worked upon. The presence ofan abrasive material, however, is common to all CMP slurries.

Conventional abrasive powders are hard agglomerates (i.e. aggregates)typically with an average aggregate size of less than about 1 μm indiameter. The particles are typically agglomerates of smaller particleshaving an irregular shape, which decreases the performance of theslurry. Larger non-spherical particles and particle agglomerates in theslurry result in scratching of the surface as well as uneven andunpredictable polishing rates. As a result, semiconductor devicemanufacturers are forced to dispose of a significant number of defectivedevices, increasing the production cost.

CMP slurries can be placed into categories based on the materials to bepolished. Oxide polishing refers to the polishing of the oxide orinterlayer dielectrics in integrated circuits, while metal polishing isthe polishing of metal interconnects (plugs) in integrated circuits.Silica (SiO₂) and alumina (Al₂O₃) are most widely used as abrasives formetal polishing, while silica is used almost exclusively for oxidepolishing. Ceria (CeO₂) is also used for some applications, includingmetal polishing and polymer polishing.

Many examples of CMP processes are illustrated in the prior art,particularly slurries for use in CMP. For example, U.S. Pat. No.4,910,155 by Cote et al. discloses a chemical mechanical polishingprocess for planarizing insulators wherein a slurry of particulates atan elevated temperature is disposed between a rotating polish pad andthe insulator surface to be polished. U.S. Pat. No. 4,954,142 by Carr etal. discloses a chemical mechanical polishing method including the useof a slurry comprising abrasive particles, a transition metal chelatedsalt and a solvent for the salt. The salt selectively etches certainfeatures, such as copper vias, on the surface of an electronic componentsubstrate.

U.S. Pat. No. 5,209,816 by Yu et al. discloses a semiconductorplanarization method particularly useful for aluminum containing metallayers. The method includes the use of a mechanical polishing slurrycomprising phosphoric acid and hydrogen peroxide. The hydrogen peroxideis an oxidizing agent that oxidizes the aluminum to alumina, which issubsequently etched by the phosphoric acid. U.S. Pat. No. 5,225,034 byYu discloses a semiconductor planarization method particularly usefulfor planarization of copper containing layers. The method utilizes aslurry which includes at least one of HNO₃, H₂SO₄ and AgNO₃.

Some work has been directed to the tailoring of the abrasive particlecomponent. For example, U.S. Pat. No. 5,264,010 by Brancaleoni et al.discloses an abrasive composition for use in planarizing the surface ofa work piece, wherein the abrasive portion includes 30 to 50 weightpercent cerium oxide, 8 to 20 weight percent fumed silica and 15 to 45weight percent precipitated silica. It is disclosed that the combinationof the three types of abrasive provides good planarization.

U.S. Pat. No. 5,527,423 by Neville et al. discloses a slurry for use inchemical mechanical polishing of metal layers. The slurry includesabrasive particles that are agglomerates of very small particles and areformed from fumed silicas or fumed aluminas. The agglomerated particles,typical of fumed materials, have a jagged, irregular shape. It isdisclosed that the particles have a surface area of 40 to 430 m²/g, anaggregate size distribution less than about 1 μm and a mean aggregatediameter of less than about 0.4 μm.

U.S. Pat. No. 5,389,352 by Wang disclosed a process for preparing achemically active oxide particle composed primarily of CeO₂. The methodincludes forming an aqueous solution of a cerium salt and an oxidizingagent and aging the mixture for at least about 4 hours. The particlesize of the powder was on the order of about 0.1 μm. U.S. Pat. No.5,429,647 by Larmie discloses a method for preparing an abrasive grainwhich includes both alumina and ceria.

U.S. Pat. No. 5,693,239 by Wang et al. discloses a CMP slurry whichincludes abrasive particles wherein about 1 to 50 weight percent of theparticles and crystalline alumina and the remainder of the particles areless abrasive materials such as aluminum hydroxides, silica and thelike.

Each of the foregoing U.S. patents is incorporated herein by reference,in their entirety.

Conventional CMP powders typically consist of agglomerated particleshaving an agglomerate shape that is inadequate for reproducible CMPperformance. As a result, integrated circuit manufacturers scrapsignificant amounts of product due to defects formed during the CMPstep. Furthermore, as integrated circuit dimensions continue to decreasein size, there will be an increasing demand for higher CMP performanceand reliability, particularly for damascene and dual damascenestructures.

Conventional CMP powders are produced using a flame combustion processwhich produces small particles that are highly agglomerated. Theseparticles are commonly referred to as fumed. Due to the high degree ofagglomeration, these particles have angular and irregular shapes whichlowers the reliability of the CMP process by virtue of uncontrollablepolishing and removal rates. The elongated and angular particles behavelike miniature knife blades cutting into the surface of the integratedcircuit. Non-spherical particles also tend to abrade the metal portionsat too high of an abrasion rate.

There is a need for abrasive powders for CMP slurries that will permitbetter control over the planarization process. It would be particularlyadvantageous if such powders could be produced in large quantities on acontinuous basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view of a furnace and showing one embodiment of thepresent invention for sealing the end of a furnace tube.

FIG. 3 is a view of the side of an end cap that faces away from thefurnace shown in FIG. 2.

FIG. 4 is a view of the side of an end cap that faces toward the furnaceshown in FIG. 2.

FIG. 5 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 6 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 7 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 8 is a side view of the transducer mounting plate shown in FIG. 7.

FIG. 9 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 7.

FIG. 10 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 11 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 12 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 13 is a side view of the liquid feed box shown in FIG. 8.

FIG. 14 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 15 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 16 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 17 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 18 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 19 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 18.

FIG. 20 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 21 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 22 is a side view of the gas manifold shown in FIG. 21.

FIG. 23 is atop view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 24 is a side view of the generator lid shown in FIG. 23.

FIG. 25 is a process block diagram of one embodiment in the presentinvention including an aerosol concentrator.

FIG. 26 is a top view in cross section of a virtual impactor that may beused for concentrating an aerosol according to the present invention.

FIG. 27 is a front view of an upstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 28 is a top view of the upstream plate assembly shown in FIG. 27.

FIG. 29 is a side view of the upstream plate assembly shown in FIG. 27.

FIG. 30 is a front view of a downstream plate assembly of the virtualimpactor shown in FIG. 26.

FIG. 31 is a top view of the downstream plate assembly shown in FIG. 30.

FIG. 32 is a side view of the downstream plate assembly shown in FIG.30.

FIG. 33 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 34 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 35 is a front view of a flow control plate of the impactor shown inFIG. 34.

FIG. 36 is a front view of a mounting plate of the impactor shown inFIG. 34.

FIG. 37 is a front view of an impactor plate assembly of the impactorshown in FIG. 34.

FIG. 38 is a side view of the impactor plate assembly shown in FIG. 37.

FIG. 39 shows a side view in cross section of a virtual impactor incombination with an impactor of the present invention for concentratingand classifying droplets in an aerosol.

FIG. 40 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 41 is a top view of a gas quench cooler of the present invention.

FIG. 42 is an end view of the gas quench cooler shown in FIG. 41.

FIG. 43 is a side view of a perforated conduit of the quench coolershown in FIG. 41.

FIG. 44 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 45 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 46 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIGS. 47a, 47 b, 47 c, 47 d, 47 e, and 47 f show cross sections ofvarious particle morphologies of some composite particles manufacturableaccording to the present invention.

FIG. 48 shows a side view of one embodiment of apparatus of the presentinvention including an aerosol generator, an aerosol concentrator, adroplet classifier, a furnace, a particle cooler, and a particlecollector.

FIG. 49 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

FIG. 50 illustrates a device for planarizing a substrate according to anembodiment of the present invention.

FIG. 51 illustrates the various steps in a chemical mechanicalplanarization process according to an embodiment of the presentinvention.

FIG. 52 illustrates a damascene process according to an embodiment ofthe present invention.

FIG. 53 illustrates a chemical mechanical planarization process using aconventional CMP slurry.

FIG. 54 illustrates a chemical mechanical planarization processaccording to the present invention.

FIG. 55 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 56 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 57 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 58 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 59 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 60 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 61 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 62 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 63 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 64 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 65 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 66 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

FIG. 67 illustrates a photomicrograph of CMP abrasive particlesaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to abrasive particles andmethods for making abrasive particles and slurries incorporating theabrasive particles.

In one aspect, the present invention provides a method for preparing aparticulate product. A feed of liquid-containing, flowable medium,including at least one precursor for the desired particulate product, isconverted to aerosol form, with droplets of the medium being dispersedin and suspended by a carrier gas. Liquid from the droplets in theaerosol is then removed to permit formation in a dispersed state of thedesired particles. Typically, the feed precursor is pyrolyzed in afurnace to make the particles. In one embodiment, the particles aresubjected, while still in a dispersed state, to compositional orstructural modification, if desired. Compositional modification mayinclude, for example, coating the particles. Structural modification mayinclude, for example, crystallization, recrystallization ormorphological alteration of the particles. The term powder is often usedherein to refer to the particulate product of the present invention. Theuse of the term powder does not indicate, however, that the particulateproduct must be dry or in any particular environment. Although theparticulate product is typically manufactured in a dry state, theparticulate product may, after manufacture, be placed in a wetenvironment, such as in a paste or slurry.

The process of the present invention is particularly well suited for theproduction of particulate products of finely divided particles having asmall weight average size. In addition to making particles within adesired range of weight average particle size, with the presentinvention the particles may be produced with a desirably narrow sizedistribution, thereby providing size uniformity that is desired for manyapplications.

In addition to control over particle size and size distribution, themethod of the present invention provides significant flexibility forproducing particles of varying composition, crystallinity andmorphology. For example, the present invention may be used to producehomogeneous particles involving only a single phase or multi-phaseparticles including multiple phases. In the case of multi-phaseparticles, the phases may be present in a variety of morphologies. Forexample, one phase may be uniformly dispersed throughout a matrix ofanother phase. Alternatively, one phase may form an interior core whileanother phase forms a coating that surrounds the core. Othermorphologies are also possible, as discussed more fully below.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including at least oneprecursor for the desired particles, and a carrier gas 104 are fed to anaerosol generator 106 where an aerosol 108 is produced. The aerosol 108is then fed to a furnace 110 where liquid in the aerosol 108 is removedto produce particles 112 that are dispersed in and suspended by gasexiting the furnace 110. The particles 112 are then collected in aparticle collector 114 to produce a particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, those particles should be relatively smallin relation to the size of droplets in the aerosol 108. Such suspendedparticles should typically be smaller than about 1 μm in size,preferably smaller than about 0.5 μm in size, and more preferablysmaller than about 0.3 μm in size and most preferably smaller than about0.1 μm in size. Most preferably, the suspended particles should be ableto form a colloid. The suspended particles could be finely dividedparticles, or could be agglomerate masses comprised of agglomeratedsmaller primary particles. For example, 0.5 μm particles could beagglomerates of nanometer-sized primary particles. When the liquid feed102 includes suspended particles, the particles typically comprise nogreater than about 25 to 50 weight percent of the liquid feed.

As noted, the liquid feed 102 includes at least one precursor forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The precursor may undergo one or morechemical reactions in the furnace 110 to assist in production of theparticles 112. Alternatively, the precursor material may contribute toformation of the particles 112 without undergoing chemical reaction.This could be the case, for example, when the liquid feed 102 includes,as a precursor material, suspended particles that are not chemicallymodified in the furnace 110. In any event, the particles 112 comprise atleast one component originally contributed by the precursor.

The liquid feed 102 may include multiple precursor materials, which maybe present together in a single phase or separately in multiple phases.For example, the liquid feed 102 may include multiple precursors insolution in a single liquid vehicle. Alternatively, one precursormaterial could be in a solid particulate phase and a second precursormaterial could be in a liquid phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case when the liquid feed 102comprises an emulsion. Different components contributed by differentprecursors may be present in the particles together in a single materialphase, or the different components may be present in different materialphases when the particles 112 are composites of multiple phases.Specific examples of preferred precursor materials are discussed morefully below.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form.Also, the carrier gas 104 may be inert, in that the carrier gas 104 doesnot participate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the particles 112. Preferred carrier gascompositions are discussed more fully below.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may, however, also include nonliquid material, such asone or more small particles held in the droplet by the liquid. Forexample, when the particles 112 are composite, or multi-phase,particles, one phase of the composite may be provided in the liquid feed102 in the form of suspended precursor particles and a second phase ofthe composite may be produced in the furnace 110 from one or moreprecursors in the liquid phase of the liquid feed 102. Furthermore theprecursor particles could be included in the liquid feed 102, andtherefore also in droplets of the aerosol 108, for the purpose only ofdispersing the particles for subsequent compositional or structuralmodification during or after processing in the furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size, narrow sizedistribution. In this manner, the particles 112 may be produced at adesired small size with a narrow size distribution, which areadvantageous for many applications.

The aerosol generator 106 is capable of producing the aerosol 108 suchthat it includes droplets having a weight average size in a range havinga lower limit of about 1 μm and preferably about 2 μm; and an upperlimit of about 10 μm; preferably about 7 μm, more preferably about 5 μmand most preferably about 4 μm. A weight average droplet size in a rangeof from about 2 μm to about 4 μm is more preferred for mostapplications, with a weight average droplet size of about 3 μm beingparticularly preferred for some applications. The aerosol generator isalso capable of producing the aerosol 108 such that it includes dropletsin a narrow size distribution. Preferably, the droplets in the aerosolare such that at least about 70 percent (more preferably at least about80 weight percent and most preferably at least about 85 weight percent)of the droplets are smaller than about 10 μm and more preferably atleast about 70 weight percent (more preferably at least about 80 weightpercent and most preferably at least about 85 weight percent) aresmaller than about 5 μm. Furthermore, preferably no greater than about30 weight percent, more preferably no greater than about 25 weightpercent and most preferably no greater than about 20 weight percent, ofthe droplets in the aerosol 108 are larger than about twice the weightaverage droplet size.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 μm to about 4 μm, the droplet loading is preferablylarger than about 0.15 milliliters of aerosol feed 102 per liter ofcarrier gas 104, more preferably larger than about 0.2 milliliters ofliquid feed 102 per liter of carrier gas 104, even more preferablylarger than about 0.2 milliliters of liquid feed 102 per liter ofcarrier gas 104, and most preferably larger than about 0.3 millilitersof liquid feed 102 per liter of carrier gas 104. When reference is madeherein to liters of carrier gas 104, it refers to the volume that thecarrier gas 104 would occupy under conditions of standard temperatureand pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the particles 112. The maximum average streamtemperature, or reaction temperature, refers to the maximum averagetemperature that an aerosol stream attains while flowing through thefurnace. This is typically determined by a temperature probe insertedinto the furnace. Preferred reaction temperatures according to thepresent invention are discussed more fully below.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum stream temperature for a given heat transfer rate. In thatregard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range. That mode of operation, however, is notpreferred. Also, it is preferred that, in most cases, the maximum streamtemperature not be attained in the furnace 110 until substantially atthe end of the heating zone in the furnace 110. For example, the heatingzone will often include a plurality of heating sections that are eachindependently controllable. The maximum stream temperature shouldtypically not be attained until the final heating section, and morepreferably until substantially at the end of the last heating section.This is important to reduce the potential for thermophoretic losses ofmaterial. Also, it is noted that as used herein, residence time refersto the actual time for a material to pass through the relevant processequipment. In the case of the furnace, this includes the effect ofincreasing velocity with gas expansion due to heating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Moreimportant, however, the accumulation of liquid at sharp edges can resultin re-release of undesirably large droplets back into the aerosol 108,which can cause contamination of the particulate product 116 withundesirably large particles. Also, over time, such liquid collection atsharp surfaces can cause fouling of process equipment, impairing processperformance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Alternatively, the tube may be metallic.Advantages of using a metallic tube are low cost, ability to withstandsteep temperature gradients and large thermal shocks, machinability andweldability, and ease of providing a seal between the tube and otherprocess equipment. Disadvantages of using a metallic tube includelimited operating temperature and increased reactivity in some reactionsystems.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube Is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture.

When higher temperatures are required, ceramic tubes are typically used.One major problem with ceramic tubes, however, is that the tubes can bedifficult to seal with other process equipment, especially when the endsof the tubes are maintained at relatively high temperatures, as is oftenthe case with the present invention.

One configuration for sealing a ceramic tube is shown in FIGS. 2, 3 and4. The furnace 110 includes a ceramic tube 374 having an end cap 376fitted to each end of the tube 374, with a gasket 378 disposed betweencorresponding ends of the ceramic tube 374 and the end caps 376. Thegasket may be of any suitable material for sealing at the temperatureencountered at the ends of the tubes 374. Examples of gasket materialsfor sealing at temperatures below about 250° C. include silicone,TEFLON™ and VITON™. Examples of gasket materials for higher temperaturesinclude graphite, ceramic paper, thin sheet metal, and combinationsthereof.

Tension rods 380 extend over the length of the furnace 110 and throughrod holes 382 through the end caps 376. The tension rods 380 are held intension by the force of springs 384 bearing against bearing plates 386and the end caps 376. The tube 374 is, therefore, in compression due tothe force of the springs 384. The springs 384 may be compressed anydesired amount to form a seal between the end caps 376 and the ceramictube 374 through the gasket 378. As will be appreciated, by using thesprings 384, the tube 374 is free to move to some degree as it expandsupon heating and contracts upon cooling. To form the seal between theend caps 376 and the ceramic tube 374, one of the gaskets 378 is seatedin a gasket seat 388 on the side of each end cap 376 facing the tube374. A mating face 390 on the side of each of the end caps 376 facesaway from the tube 374, for mating with a flange surface for connectionwith an adjacent piece of equipment.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from gas. Such a filter may be of any type,including a bag filter. Another preferred embodiment of the particlecollector uses one or more cyclone to separate the particles 112. Otherapparatus that may be used in the particle collector 114 includes anelectrostatic precipitator. Also, collection should normally occur at atemperature above the condensation temperature of the gas stream inwhich the particles 112 are suspended. Also, collection should normallybe at a temperature that is low enough to prevent significantagglomeration of the particles 112.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 5, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 5, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 6, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 5, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 5, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHz to about 5 MHz, more preferably from about 1.5 MHzto about 3 MHz, and even more preferably at a frequency greater thanabout 1.7 MHz. A particularly preferred frequency is about 2.4 MHz.Furthermore, all of the transducer discs 110 should be operating atsubstantially the same frequency when an aerosol with a narrow dropletsize distribution is desired. This is important because commerciallyavailable transducers can vary significantly in thickness, sometimes byas much as 10%. It is preferred, however, that the transducer discs 120operate at frequencies within a range of 5% above and below the mediantransducer frequency, more preferably within d range of 2.5%, and mostpreferably within a range of 1%. This can be accomplished by carefulselection of the transducer discs 120 so that they all preferably havethicknesses within 5% of the median transducer thickness, morepreferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 5 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.5, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126 in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extending the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer disks 120 are preferably also at an elevatedtemperature in the ranges just discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 7-24 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 7 and8, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays of 100 ultrasonic transducers each. The transducer mountingplate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 5.

As shown in FIGS. 7 and 8, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 9,the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 5.

A preferred transducer mounting configuration, however, is shown in FIG.10 for another configuration for the transducer mounting plate 124. Asseen in FIG. 10, an ultrasonic transducer disc 120 is mounted to thetransducer mounting plate 124 by use of a compression screw 177 threadedinto a threaded receptacle 179. The compression screw 177 bears againstthe ultrasonic transducer disc 120, causing an o-ring 181, situated inan o-ring seat 182 on the transducer mounting plate, to be compressed toform a seal between the transducer mounting plate 124 and the ultrasonictransducer disc 120. This type of transducer mounting is particularlypreferred when the ultrasonic transducer disc 120 includes a protectivesurface coating, as discussed previously, because the seal of the o-ringto the ultrasonic transducer disc 120 will be inside of the outer edgeof the protective seal, thereby preventing liquid from penetrating underthe protective surface coating from the edges of the ultrasonictransducer disc 120.

Referring now to FIG. 11, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 7-8). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 7 and 8) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 12 and 13, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 11), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 12 and 13 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 11). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 12-14, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 14. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 15, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 15are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 15,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 15, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 5, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 15 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 16, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 14. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.17. As shown in FIG. 17, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 18 and 19. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 19,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 18 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 20, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 21 and 22, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 14). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 21 and 22, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 23 and 24, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 12 and 13). The generator lid140, as shown in FIGS. 23 and 24, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

Although the aerosol generator 106 produces a high quality aerosol 108having a high droplet loading, it is often desirable to furtherconcentrate the aerosol 108 prior to introduction into the furnace 110.Referring now to FIG. 25, a process flow diagram is shown for oneembodiment of the present invention involving such concentration of theaerosol 108. As shown in FIG. 25, the aerosol 108 from the aerosolgenerator 106 is sent to an aerosol concentrator 236 where excesscarrier gas 238 is withdrawn from the aerosol 108 to produce aconcentrated aerosol 240, which is then fed to the furnace 110.

The aerosol concentrator 236 typically includes one or more virtualimpactors capable of concentrating droplets in the aerosol 108 by afactor of greater than about 2, preferably by a factor of greater thanabout 5, and more preferably by a factor of greater than about 10 toproduce the concentrated aerosol 240. According to the presentinvention, the concentrated aerosol 240 should typically contain greaterthan about 1×10⁷ droplets per cubic centimeter, and more preferably fromabout 5×10⁷ to about 5×10⁸ droplets per cubic centimeter. Aconcentration of about 1×10⁸ droplets per cubic centimeter of theconcentrated aerosol is particularly preferred, because when theconcentrated aerosol 240 is loaded more heavily than that, then thefrequency of collisions between droplets becomes large enough to impairthe properties of the concentrated aerosol 240, resulting in potentialcontamination of the particulate product 116 with an undesirably largequantity of over-sized particles. For example, if the aerosol 108 has aconcentration of about 1×10⁷ droplets per cubic centimeter, and theaerosol concentrator 236 concentrates droplets by a factor of 10, thenthe concentrated aerosol 240 will have a concentration of about 1×10⁸droplets per cubic centimeter. Stated another way, for example, when theaerosol generator generates the aerosol 108 with a droplet loading ofabout 0.167 milliliters liquid teed 102 per liter of carrier gas 104,the concentrated aerosol 240 would be loaded with about 1.67 millilitersof liquid feed 102 per liter of carrier gas 104, assuming the aerosol108 is concentrated by a factor of 10.

Having a high droplet loading in aerosol feed to the furnace providesthe important advantage of reducing the heating demand on the furnace110 and the size of flow conduits required through the furnace. Also,other advantages of having a dense aerosol include a reduction in thedemands on cooling and particle collection components, permittingsignificant equipment and operational savings. Furthermore, as systemcomponents are reduced in size, powder holdup within the system isreduced, which is also desirable. Concentration of the aerosol streamprior to entry into the furnace 110, therefore, provides a substantialadvantage relative to processes that utilize less concentrated aerosolstreams.

The excess carrier gas 238 that is removed in the aerosol concentrator236 typically includes extremely small droplets that are also removedfrom the aerosol 108. Preferably, the droplets removed with the excesscarrier gas 238 have a weight average size of smaller than about 1.5 μm,and more preferably smaller than about 1 μm and the droplets retained inthe concentrated aerosol 240 have an average droplet size of larger thanabout 2 μm. For example, a virtual impactor sized to treat an aerosolstream having a weight average droplet size of about 3 μm might bedesigned to remove with the excess carrier gas 238 most droplets smallerthan about 1.5 μm in size. Other designs are also possible. When usingthe aerosol generator 106 with the present invention, however, the lossof these very small droplets in the aerosol concentrator 236 willtypically constitute no more than about 10 percent by weight, and morepreferably no more than about 5 percent by weight, of the dropletsoriginally in the aerosol stream that is fed to the concentrator 236.Although the aerosol concentrator 236 is useful in some situations, itis normally not required with the process of the present invention,because the aerosol generator 106 is capable, in most circumstances, ofgenerating an aerosol stream that is sufficiently dense. So long as theaerosol stream coming out of the aerosol generator 102 is sufficientlydense, it is preferred that the aerosol concentrator not be used. It isa significant advantage of the present invention that the aerosolgenerator 106 normally generates such a dense aerosol stream that theaerosol concentrator 236 is not needed. Therefore, the complexity ofoperation of the aerosol concentrator 236 and accompanying liquid lossesmay typically be avoided.

It is important that the aerosol stream (whether it has beenconcentrated or not) that is fed to the furnace 110 have a high dropletflow rate and high droplet loading as would be required for mostindustrial applications. With the present invention, the aerosol streamfed to the furnace preferably includes a droplet flow of greater thanabout 0.5 liters per hour, more preferably greater than about 2 litersper hour, still more preferably greater than about 5 liters per hour,even more preferably greater than about 10 liters per hour, particularlygreater than about 50 liters per hour and most preferably greater thanabout 100 liters per hour; and with the droplet loading being typicallygreater than about 0.04 milliliters of droplets per liter of carriergas, preferably greater than about 0.083 milliliters of droplets perliter of carrier gas 104, more preferably greater than about 0.167milliliters of droplets per liter of carrier gas 104, still morepreferably greater than about 0.25 milliliters of droplets per liter ofcarrier gas 104, particularly greater than about 0.33 milliliters ofdroplets per liter of carrier gas 104 and most preferably greater thanabout 0.83 milliliters of droplets per liter of carrier gas 104.

One embodiment of a virtual impactor that could be used as the aerosolconcentrator 236 will now be described with reference to FIGS. 26-32. Avirtual impactor 246 includes an upstream plate assembly 248 (detailsshown in FIGS. 27-29) and a downstream plate assembly 250 (details shownin FIGS. 25-32), with a concentrating chamber 262 located between theupstream plate assembly 248 and the downstream plate assembly 250.

Through the upstream plate assembly 248 are a plurality of verticallyextending inlet slits 254. The downstream plate assembly 250 includes aplurality of vertically extending exit slits 256 that are in alignmentwith the inlet slits 254. The exit slits 256 are, however, slightlywider than the inlet slits 254. The downstream plate assembly 250 alsoincludes flow channels 258 that extend substantially across the width ofthe entire downstream plate assembly 250, with each flow channel 258being adjacent to an excess gas withdrawal port 260.

During operation, the aerosol 108 passes through the inlet slits 254 andinto the concentrating chamber 262. Excess carrier gas 238 is withdrawnfrom the concentrating chamber 262 via the excess gas withdrawal ports260. The withdrawn excess carrier gas 238 then exits via a gas duct port264. That portion of the aerosol 108 that is not withdrawn through theexcess gas withdrawal ports 260 passes through the exit slits 256 andthe flow channels 258 to form the concentrated aerosol 240. Thosedroplets passing across the concentrating chamber 262 and through theexit slits 256 are those droplets of a large enough size to havesufficient momentum to resist being withdrawn with the excess carriergas 238.

As seen best in FIGS. 27-32, the inlet slits 254 of the upstream plateassembly 248 include inlet nozzle extension portions 266 that extendoutward from the plate surface 268 of the upstream plate assembly 248.The exit slits 256 of the downstream plate assembly 250 include exitnozzle extension portions 270 extending outward from a plate surface 272of the downstream plate assembly 250. These nozzle extension portions266 and 270 are important for operation of the virtual impactor 246,because having these nozzle extension portions 266 and 270 permits avery close spacing to be attained between the inlet slits 254 and theexit slits 256 across the concentrating chamber 262, while alsoproviding a relatively large space in the concentrating chamber 262 tofacilitate efficient removal of the excess carrier gas 238

Also as best seen in FIGS. 27-32, the inlet slits 254 have widths thatflare outward toward the side of the upstream plate assembly 248 that isfirst encountered by the aerosol 108 during operation. This flaredconfiguration reduces the sharpness of surfaces encountered by theaerosol 108, reducing the loss of aerosol droplets and potentialinterference from liquid buildup that could occur if sharp surfaces werepresent. Likewise, the exit slits 256 have a width that flares outwardtowards the flow channels 258, thereby allowing the concentrated aerosol240 to expand into the flow channels 258 without encountering sharpedges that could cause problems.

As noted previously, both the inlet slits 254 of the upstream plateassembly 248 and the exit slits 256 of the downstream plate assembly 250are vertically extending. This configuration is advantageous forpermitting liquid that may collect around the inlet slits 254 and theexit slits 256 to drain away. The inlet slits 254 and the exit slits 256need not, however, have a perfectly vertical orientation. Rather, it isoften desirable to slant the slits backward (sloping upward and away inthe direction of flow) by about five to ten degrees relative tovertical, to enhance draining of liquid off of the upstream plateassembly 248 and the downstream plate assembly 250. This drainagefunction of the vertically extending configuration of the inlet slits254 and the outlet slits 256 also inhibits liquid build-up In thevicinity of the inlet slits 248 and the exit slits 250, which liquidbuild-up could result in the release of undesirably large droplets intothe concentrated aerosol 240.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

Referring now to FIG. 33, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 33, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 34-38.

As seen in FIG. 34, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 Is shown in FIG. 35. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 36. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 35).

Referring now to FIGS. 37 and 38, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 34).

The configuration of the impactor plate 302 shown in FIG. 33 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 μm in size, more preferably to removedroplets larger than about 10 μm in size, even more preferably to removedroplets of a size larger than about 8 μm in size and most preferably toremove droplets larger than about 5 μm in size. The dropletclassification size in the droplet classifier is preferably smaller thanabout 15 μm, more preferably smaller than about 10 μm, even morepreferably smaller than about 8 μm and most preferably smaller thanabout 5 μm. The classification size, also called the classification cutpoint, is that size at which half of the droplets of that size areremoved and half of the droplets of that size are retained. Dependingupon the specific application, however, the droplet classification sizemay be varied, such as by changing the spacing between the impactorplate 302 and the flow control plate 290 or increasing or decreasingaerosol velocity through the jets in the flow control plate 290. Becausethe aerosol generator 106 of the present invention initially produces ahigh quality aerosol 108, having a relatively narrow size distributionof droplets, typically less than about 30 weight percent of liquid feed102 in the aerosol 108 is removed as the drain liquid 284 in the dropletclassifier 288, with preferably less than about 25 weight percent beingremoved, even more preferably less than about 20 weight percent beingremoved and most preferably less than about 15 weight percent beingremoved. Minimizing the removal of liquid feed 102 from the aerosol 108is particularly important for commercial applications to increase theyield of high quality particulate product 116. It should be noted,however, that because of the superior performance of the aerosolgenerator 106, it is frequently not required to use an impactor or otherdroplet classifier to obtain a desired absence of oversize droplets tothe furnace. This is a major advantage, because the added complexityarid liquid losses accompanying use of an impactor may often be avoidedwith the process of the present invention.

Sometimes it is desirable to use both the aerosol concentrator 236 andthe droplet classifier 280 to produce an extremely high quality aerosolstream for introduction into the furnace for the production of particlesof highly controlled size and size distribution. Referring now to FIG.39, one embodiment of the present invention is shown incorporating boththe virtual impactor 246 and the impactor 288. Basic components of thevirtual impactor 246 and the impactor 288, as shown in FIG. 39, aresubstantially as previously described with reference to FIGS. 26-38. Asseen in FIG. 39, the aerosol 108 from the aerosol generator 106 is fedto the virtual impactor 246 where the aerosol stream is concentrated toproduce the concentrated aerosol 240. The concentrated aerosol 240 isthen fed to the impactor 288 to remove large droplets therefrom andproduce the classified aerosol 282, which may then be fed to the furnace110. Also, it should be noted that by using both a virtual impactor andan impactor, both undesirably large and undesirably small droplets areremoved, thereby producing a classified aerosol with a very narrowdroplet size distribution. Also, the order of the aerosol concentratorand the aerosol classifier could be reversed, so that the aerosolconcentrator 236 follows the aerosol classifier 280.

One important feature of the design shown in FIG. 39 is theincorporation of drains 310, 312, 314, 316 and 296 at strategiclocations. These drains are extremely important for industrial-scaleparticle production because buildup of liquid in the process equipmentcan significantly impair the quality of the particulate product 116 thatis produced. In that regard, drain 310 drains liquid away from the inletside of the first plate assembly 248 of the virtual impactor 246. Drain312 drains liquid away from the inside of the concentrating chamber 262in the virtual impactor 246 and drain 314 removes liquid that depositsout of the excess carrier gas 238. Drain 316 removes liquid from thevicinity of the inlet side of the flow control plate 290 of theimpactor, while the drain 296 removes liquid from the vicinity of theimpactor plate 302. Without these drains 310, 312, 314, 316 and 296, theperformance of the apparatus shown in FIG. 39 would be significantlyimpaired. All liquids drained in the drains 310, 312, 314, 316 and 296may advantageously be recycled for use to prepare the liquid feed 102.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 40,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then feed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 41-43, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 43, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 41-43, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 41, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 41-43, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example when the particle collectorincludes a cyclone for separating particles in the cooled particlestream 322 from gas in the cooled particle stream 322. This is becauseof the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 44, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 μm, although a series of cyclones maysometimes be needed to get the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 μm. Also, cyclone separation is bestsuited for high density materials. Preferably, when particles areseparated using a cyclone, the particles are of a composition withspecific gravity of greater than about 5.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the particles 112 exitingthe furnace. Most commonly, the compositional modification will involveforming on the particles 112 a material phase that is different thanthat of the particles 112, such as by coating the particles 112 with acoating material. One embodiment of the process of the present inventionincorporating particle coating is shown in FIG. 45. As shown in FIG. 45,the particles 112 exiting from the furnace 110 go to a particle coater350 where a coating is placed over the outer surface of the particles112 to form coated particles 352, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Coatingmethodologies employed in the particle coater 350 are discussed in moredetail below.

With continued reference primarily to FIG. 45, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process, such asby a liquid precipitation route. When coating particles that have beenpremanufactured by a different route, such as by liquid precipitation,it is preferred that the particles remain in a dispersed state from thetime of manufacture to the time that the particles are introduced inslurry form into the aerosol generator 106 for preparation of theaerosol 108 to form the dry particles 112 in the furnace 110, whichparticles 112 can then be coated in the particle coater 350. Maintainingparticles in a dispersed state from manufacture through coating avoidsproblems associated with agglomeration and redispersion of particles ifparticles must be redispersed in the liquid feed 102 for feed to theaerosol generator 106. For example, for particles originallyprecipitated from a liquid medium, the liquid medium containing thesuspended precipitated particles could be used to form the liquid feed102 to the aerosol generator 106. It should be noted that the particlecoater 350 could be an integral extension of the furnace 110 or could bea separate piece of equipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to Impart desired physical properties prior toparticle collection. Referring now to FIG. 46, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112 into a polycrystalline or single crystalline form.Also, especially in the case of composite particles 112, the particlesmay be annealed for a sufficient time to permit redistribution withinthe particles 112 of different material phases. Particularly preferredparameters for such processes are discussed in more detail below.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 47. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 47.

Referring now to FIG. 48, an embodiment of the apparatus of the presentinvention is shown that includes the aerosol generator 106 (in the formof the 400 transducer array design), the aerosol concentrator 236 (inthe form of a virtual impactor), the droplet classifier 280 (in the formof an impactor), the furnace 110, the particle cooler 320 (in the formof a gas quench cooler) and the particle collector 114 (in the form of abag filter). All process equipment components are connected viaappropriate flow conduits that are substantially free of sharp edgesthat could detrimentally cause liquid accumulations in the apparatus.Also, it should be noted that flex connectors 370 are used upstream anddownstream of the aerosol concentrator 236 and the droplet classifier280. By using the flex connectors 370, it is possible to vary the angleof slant of vertically extending slits in the aerosol concentrator 236and/or the droplet classifier 280. In this way, a desired slant for thevertically extending slits may be set to optimize the drainingcharacteristics off the vertically extending slits.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generation for theprocess of the present invention may have a different design dependingupon the specific application. For example, when larger particles aredesired, such as those having a weight average size of larger than about3 μm, a spray nozzle atomizer may be preferred. For smaller-particleapplications, however, and particularly for those applications toproduce particles smaller than about 3 μm, and preferably smaller thanabout 2 μm in size, as is generally desired with the particles of thepresent invention, the ultrasonic generator, as described herein, isparticularly preferred. In that regard, the ultrasonic generator of thepresent invention Is particularly preferred for when making particleswith a weight average size of from about 0.2 μm to about 3 μm.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 5-24,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof inadequately narrow size distribution, undesirably low dropletloading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 μm toabout 10 μm, preferably in a size range of from about 1 μm to about 5 μmand more preferably from about 2 μm to about 4 μm. Also, the ultrasonicgenerator of the present invention is capable of delivering high outputrates of liquid feed in the aerosol. The rate of liquid feed, at thehigh liquid loadings previously described, is preferably greater thanabout 25 milliliters per hour per transducer, more preferably greaterthan about 37.5 milliliters per hour per transducer, even morepreferably greater than about 50 milliliters per hour per transducer andmost preferably greater than about 100 millimeters per hour pertransducer. This high level of performance is desirable for commercialoperations and is accomplished with the present invention with arelatively simple design including a single precursor bath over an arrayof ultrasonic transducers. The ultrasonic generator is made for highaerosol production rates at a high droplet loading, and with a narrowsize distribution of droplets. The generator preferably produces anaerosol at a rate of greater than about 0.5 liter per hour of droplets,more preferably greater than about 2 liters per hour of droplets, stillmore preferably greater than about 5 liters per hour of droplets, evenmore preferably greater than about 10 liters per hour of droplets andmost preferably greater than about 40 liters per hour of droplets. Forexample, when the aerosol generator has a 400 transducer design, asdescribed with reference to FIGS. 7-24, the aerosol generator is capableof producing a high quality aerosol having high droplet loading aspreviously described, at a total production rate of preferably greaterthan about 10 liters per hour of liquid feed, more preferably greaterthan about 15 liters per hour of liquid feed, even more preferablygreater than about 20 liters per hour of liquid feed and most preferablygreater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as: ${Re} = \frac{\rho \quad v\quad d}{\mu}$ $\begin{matrix}{{{\text{where:}\quad \rho} = {{fluid}\quad {density}}};} \\{\quad {{v = {{fluid}\quad {mean}\quad {velocity}}};}} \\{\quad {{d = {{conduit}\quad {inside}\quad {diameter}}};\quad {and}}} \\{\quad {\mu = {{fluid}{\quad \quad}{{viscosity}.}}}}\end{matrix}$

It should be noted that the values for density, velocity and viscositywill vary along the length of the furnace 110. The maximum Reynoldsnumber in the furnace 110 is typically attained when the average streamtemperature is at a maximum, because the gas velocity is at a very highvalue due to gas expansion when heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum stream temperature is reached in the furnace. With the presentinvention, the average residence time in the heating zone of the furnacemay typically be maintained at shorter than about 4 seconds, preferablyshorter than about 2 seconds, more preferably shorter than about 1second, still more preferably shorter than about 0.5 second and mostpreferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated. If system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum stream temperature is notattained until near the end of the heating zone in the furnace, and atleast not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,It is preferred that the residence time of the aerosol stream betweenattaining the maximum stream temperature in the furnace and a point atwhich the aerosol has been cooled to an average stream temperature belowabout 200° C. is shorter than about 2 seconds, more preferably shorterthan about 1 second, and even more preferably shorter than about 0.5second and most preferably shorter than about 0.1 second. In mostinstances, the maximum average stream temperature attained in thefurnace will be greater than about 800° C. Furthermore, the totalresidence time from the beginning of the heating zone in the furnace tod point at which the average stream temperature is at a temperaturebelow about 200° C. should typically be shorter than about 5 seconds,preferably shorter than about 3 seconds, more preferably shorter thanabout 2 seconds, and most preferably shorter than about 1 second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

In one embodiment, wash fluid used to wash particles from the interiorwalls of particle collection equipment includes a surfactant. Some ofthe surfactant will adhere to the surface of the particles. This couldbe advantageous to reduce agglomeration tendency of the particles and toenhance dispersibility of the particles in a thick film pastformulation. The surfactant could be selected for compatibility with thespecific paste formulation anticipated.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes. One way toreduce the potential for undesirable liquid buildup in the system is toprovide adequate drains, as previously described. In that regard, it ispreferred that a drain be placed as close as possible to the furnaceinlet to prevent liquid accumulations from reaching the furnace. Thedrain should be placed, however, far enough in advance of the furnaceinlet such that the stream temperature is lower than about 80° C. at thedrain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 49, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110.

The dry gas 118 will most often be dry air, although in some instancesit may be desirable to use dry nitrogen gas or some other dry gas. Ifsufficient a sufficient quantity of the dry gas 118 is used, thedroplets of the aerosol 108 are substantially completely dried tobeneficially form dried precursor particles in aerosol form forintroduction into the furnace 110, where the precursor particles arethen pyrolyzed to make a desired particulate product. Also, the use ofthe dry gas 118 typically will reduce the potential for contact betweendroplets of the aerosol and the conduit wall, especially in the criticalarea in the vicinity of the inlet to the furnace 110. In that regard, apreferred method for introducing the dry gas 118 into the aerosol 108 isfrom a radial direction into the aerosol 108. For example, equipment ofsubstantially the same design as the quench cooler, described previouslywith reference to FIGS. 41-43, could be used, with the aerosol 108flowing through the interior flow path of the apparatus and the dry gas118 being introduced through perforated wall of the perforated conduit.An alternative to using the dry gas 118 to dry the aerosol 108 would beto use a low temperature thermal preheater/dryer prior to the furnace110 to dry the aerosol 108 prior to introduction into the furnace 110.This alternative is not, however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 μm, this vertical flow should,preferably, be vertically upward. For larger-size particles, such asthose larger than about 1.5 μm, the vertical flow is preferablyvertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemretention time from the outlet of the generator until collection of theparticles is typically shorter than about 10 seconds, preferably shorterthan about 7 seconds, more preferably shorter than about 5 seconds andmost preferably shorter than about 3 seconds.

For the production of abrasives such as CMP abrasives, the liquid feed102 includes at least one abrasive particle precursor for preparation ofthe abrasive particles 112. The abrasive particle precursor may be asubstance in either a liquid or solid phase of the liquid feed 102.Typically, the abrasive particle precursor will be a compound, such as asalt, dissolved in a liquid solvent of the liquid feed 102. The abrasiveparticle precursor may undergo one or more chemical reactions in thefurnace 110 to assist in production of the abrasive particles 112.Alternatively, the abrasive particle precursor may contribute toformation of the abrasive particles 112 without undergoing chemicalreaction. This could be the case, for example, when the liquid feed 102includes suspended particles as a precursor material.

The liquid feed 102 thus includes the chemical components that will formthe abrasive particles. For example, the liquid feed 102 can comprise asolution containing metal nitrates, chlorides, sulfates, hydroxides, oroxalates that are capable of forming the desired abrasive compound. Forexample, preferred precursors for ceria (CeO₂) are ceric ammoniumnitrate and cerium nitrate and a preferred precursor for alumina (Al₂O₃)is aluminum nitrate. Nitrate salts are highly soluble in water and thesolutions maintain a low viscosity, even at high concentrations. Thesolution preferably has a precursor concentration that is unsaturated toavoid the possibility of precipitate formation. The solution preferablyincludes, for example, a precursor concentration to provide from about 1to about 50 weight percent, more preferably from about 1 to about 10weight percent, of the abrasive compound in solution. The final particlesize of the abrasive particles 112 is also influenced by the precursorconcentration. Generally, lower precursor concentrations will producesmaller particles.

Preferably, the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene or ethylene glycol, may be desirable.The use of organic solvents can lead to undesirable carbon concentrationin the abrasive particles. The pH of the aqueous-based solutions can beadjusted to alter the solubility characteristics of the precursor in thesolution.

The precursor solution can also include other additives such as acids,bases, complexing agents, sintering aids, fluxing agents, and the like.One preferred additive is urea, which can aid in the densification ofthe particles. Preferably, up to about 1 mole equivalent of urea isadded to the solution.

When the liquid feed 102 includes multiple precursors, more than one ofthe precursors may be an abrasive particle precursor, or one or more ofthe precursors may contain a component other than the abrasive particleprecursor that is contributed to the abrasive particles 112. Differentcomponents contributed by different precursors may be present in theparticles together in a single material phase, or the differentcomponents may be present in different material phases when the abrasiveparticles 112 are composites of multiple phases. To form compositeparticles, the liquid feed can include colloids, for example colloidalparticles of Al₂O₃, SiO₂, CeO₂, ZrO₂ or TiO₂.

A carrier gas 104 under controlled pressure is introduced to the aerosolgenerator to move the droplets away from the generator. The carrier gas104 may comprise any gaseous medium in which droplets produced from theliquid feed 102 may be dispersed in aerosol form. Also, the carrier gas104 may be inert, in that the carrier gas 104 does not participate information of the abrasive particles 112. Alternatively, the carrier gas104 may have one or more active component(s), such as oxygen, thatcontribute to formation of the abrasive particles 112. In that regard,the carrier gas may include one or more reactive components that reactin the furnace 110 to contribute to formation of the abrasive particles112. Examples of preferred carrier gases include reactive carrier gasessuch as air or oxygen and inert carrier gases such as argon or nitrogen.Air is a preferred carrier gas for forming abrasive particles accordingto the present invention. However, an oxygen-containing gas may not beneeded if sufficient NO_(x) is produced during pyrolysis of theprecursor.

According to the present invention, the stream temperature (reactiontemperature) in the heating zone is preferably from about 400° C. toabout 1200° C., such as from about 600° C. to about 1100° C., althoughit will be appreciated that the temperature will depend on the abrasivecompound being produced.

When the abrasive particles are coated abrasive particles, precursors tometal oxide coatings can be selected from volatile metal acetates,chlorides, alkoxides or halides. Such precursors are known to react athigh temperatures to form the corresponding metal oxides and eliminatesupporting ligands or ions. For example, SiCl₄ can be used as aprecursor to SiO₂ coatings where water vapor is present:

SiCl_(4(g))+2H₂O_((g))→SiO_(2(s))+4 HCl_((g))

SiCl₄ also is highly volatile and is a liquid at room temperature, whichmakes transport into the reactor more controllable.

Metal alkoxides can be used to produce metal oxide films by hydrolysis.The water molecules react with the alkoxide M-O bond resulting in cleanelimination of the corresponding alcohol with the formation of M-O-Mbonds:

Si(OEt)₄+2H₂O→SiO₂+4EtOH

Most metal alkoxides have a reasonably high vapor pressure and aretherefore well suited as coating precursors.

Metal acetates are also useful as coating precursors since they readilydecompose upon thermal activation by acetic anhydride elimination:

Mg(O₂CCH₃)₂→MgO+CH₃C(O)OC(O)CH₃

Metal acetates are advantageous as coating precursors since they arewater stable and are reasonably inexpensive.

Coatings can be generated on the abrasive particle surface by a numberof different mechanisms. One or more precursors can vaporize and fuse tothe hot particle surface and thermally react resulting in the formationof a thin-film coating by chemical vapor deposition (CVD). Preferredcoatings deposited by CVD include metal oxides and elemental metals.Further, the coating can be formed by physical vapor deposition (PVD)wherein a coating material physically deposits on the surface of theparticles. Preferred coatings deposited by PVD include organic materialsand elemental metals. Alternatively, the gaseous precursor can react inthe gas phase forming small particles, for example less than about 5nanometers in size, which then diffuse to the larger particle surfaceand sinter onto the surface, thus forming a coating. This method isreferred to as gas-to-particle conversion (GPC). Whether such coatingreactions occur by CVD, PVD or GPC is dependent on the reactorconditions, such as temperature, precursor partial pressure, waterpartial pressure and the concentration of particles in the gas stream.Another possible surface coating method is surface conversion of thesurface of the particles by reaction with a vapor phase reactant toconvert the surface of the particles to a different material than thatoriginally contained in the particles.

In addition, a volatile coating material such as PbO, MoO₃ or V₂O₅ canbe introduced into the reactor such that the coating deposits on theparticles by condensation. Highly volatile metals can also be depositedby condensation. Further, the particles can be coated using othertechniques. For example, a soluble precursor to both the abrasive powderand the coating can be used in the precursor solution wherein thecoating precursor is involatile, (e.g. Al(NO₃)₃ or volatile (e.g.Sn(OAc)₄ where Ac is acetate). In another embodiment, a colloidalprecursor and a soluble nickel precursor can be used to form aparticulate colloidal coating on the nickel particle. It will beappreciated that multiple coatings can be deposited on the surface ofthe abrasive particles if such multiple coatings are desirable.

The present invention is directed to abrasive powder batches that areparticularly useful in CMP slurries, wherein the abrasive particlesconstituting the powder batch have a spherical morphology, a smallaverage particle size and a narrow particle size distribution. Theparticles constituting the powder batch are also substantiallyunagglomerated. The powders of the present invention offer numerousadvantages over prior art abrasive particles that do not meet thesecriteria. The slurries according to the present invention areparticularly useful for the chemical-mechanical planarization ofintegrated circuit components, including dielectric (oxide) layers,referred to as ILD (interlayer dielectrics) and metal film layers, aswell as flat panel display screens.

The abrasive powder batches according to the present invention include acommercially useful quantity of abrasive particles. The abrasiveparticles according to the present invention preferably includenon-metallic compounds which have a hardness equal to or greater thanthe surface being polished. Thus, the abrasive particles can be abrasivecompounds such as oxides, carbides, nitrides, silicides and borides.Preferred abrasive compounds for use in CMP slurries are simple andcomplex metal oxides. For example, the abrasive compound can be selectedfrom complex oxides such as YAG (yttrium aluminum garnet) and glassessuch as borosilicate glasses.

Particularly preferred abrasive compounds according to present inventionare binary metal oxides. Oxides such as alumina (Al₂O₃), silica (SiO₂),titania (TiO₂) magnesia (MgO) and hafnia (HfO₂) are mechanically hardand have good abrasive qualities and alumina and silica are particularlypreferred for CMP slurries used in integrated circuit manufacture.Complex oxides, such as YAG (Y₃Al₅O₁₂) can also be useful as an abrasivecompound. Abrasive compounds which can accelerate the removal ofsubstrate material through chemical activity include ceria (CeO₂),zirconia (ZrO₂), manganese dioxide (MnO₂), vanadium oxide (V₂O₅), tinoxide (SnO₂), zinc oxide (ZnO), iron oxide (Fe₂O₃/Fe₃O₄), and chromiumoxide (Cr₂O₃). Such materials contribute to the chemistry of the slurryto accelerate the material removal rate.

For example, a preferred abrasive compound that is chemically active isceria. Ceria is commonly used for polishing technical glasses. Ceria ischemically active under typical slurry conditions:

2CeO₂+2e→Ce₂O₃+O²⁻

Thus, ceria can accelerate the removal of silica by chemically reactingand bonding with the silica surface. Ceria abrasives are able to reducethe silica and bond with the surface being treated. Bonding between theabrasive and atoms at the surface increases the shearing force of theabrasive particle, increasing the probability that material will beremoved from the surface. Further, since the abraded material remainsbonded to the abrasive particle, the probability that the abradedmaterial will be removed from the vicinity of the surface increases. Asa result, an abrasive particle such as ceria will yield higher removalrates than abrasives that do not exhibit this chemical action.

Particularly preferred abrasive compounds according to the presentinvention are SiO₂, Al₂O₃, CeO₂ and ZrO₂.

According to one embodiment of the present invention, the abrasivepowder comprises particles that have luminescent properties such thatthe particles can be detected under the application of light or otherenergy source. Thus, the abrasive particles include a luminescent orphosphorescent compound associated therewith. In a preferred embodiment,the abrasive particles include a phosphor compound. Contamination of apolished IC wafer by the abrasive particles after polishing is a seriousproblem for IC manufacturers. Abrasive particles having luminescentproperties would enable the detection of residual particles by simplyviewing the polished IC under ultraviolet or infrared light. Preferredphosphor compounds are photoluminescent or up-converter phosphors, whichare capable of emitting visible light upon the application of UV orinfrared radiation. The particles could then be detected using aphotodetector array, such as a CCD camera. According to this embodiment,the abrasive particles preferably comprise a phosphor compound such asY₂O₃ doped with an two activator ion, such as a rare-earth element (e.g.Eu). The abrasive particle can consist entirely of the phosphorcompound, or, as is discussed below, may be a composite particle of anabrasive compound such as SiO₂ with a luminescent compound as a secondphase or an abrasive particle coated with a luminescent compound, suchas a phosphor.

It will be appreciated that the abrasive powder batches and thepolishing slurries formed using the powder batches can include abrasiveparticles of one or more of the foregoing abrasive compounds. For manyapplications it will be desirable to include at least two such compoundsin the same polishing slurry, such as CeO₂ and SiO₂, to achieve acombination of chemical and mechanical activity during polishing.

The abrasive powders according to the present invention includeparticles having a small average particle size. Although the preferredaverage size of the particles will vary according to the particular CMPapplication, the weight average particle size of the abrasive particlesis preferably at least about 0.05 μm and more preferably is at leastabout 0.1 μm. Further, the weight average particle size is preferablynot greater than about 3 μm. For most applications, the weight averageparticle size is more preferably not greater than about 2 μm and evenmore preferably is not greater than about 1 μm. According to onepreferred embodiment, the weight average particle size is from about 0.1μm to about 0.75 μm. Abrasive powder batches having such an averageparticle size are particularly useful in CMP slurries for planarizingintegrated circuit layers.

According to a preferred embodiment of the present invention, the powderbatch of abrasive particles has a narrow particle size distribution,such that the majority of particles are about the same size. Preferably,at least about 90 weight percent and more preferably at least about 95weight percent of the particles are not larger than twice the weightaverage particle size. Thus, when the average particle size is about 0.5μm, it is preferred that at least about 90 weight percent of theparticles are not larger than 1 μm and it is more preferred that atleast about 95 weight percent of the particles are not larger than 1 μm.Further, it is preferred that at least about 90 weight percent and morepreferably at least about 95 weight percent of the particles are notlarger than about 1.5 times the weight average particle size. Thus, whenthe average particle size is about 0.5 μm, it is preferred that at leastabout 90 weight percent of the particles are not larger than 0.75 μm andit is more preferred that at least about 95 weight percent of theparticles are not larger than 0.75 μm.

Such a narrow size distribution of abrasive particles advantageouslyproduces a CMP slurry with increased reliability and reproducibility.The number of defects due to large abrasive particles will be reduced.Such a CMP slurry will save manufacturers of IC devices the costsassociated with low yields of acceptable devices and will reduce thetime required to planarize a substrate.

It is also possible according to the present invention to provide anabrasive powder batch having a bimodal particle size distribution. Thatis, the powder batch can include abrasive particles having two differentand distinct average particle sizes, each with a narrow sizedistribution as discussed above. In one embodiment, the powder batchincludes two different abrasive particles, having different averageparticle sizes.

The abrasive particles of the present invention can be substantiallysingle crystal particles or may be comprised of a number ofcrystallites. The particles can also be amorphous. The methodology ofthe present invention advantageously permits control over thecrystallinity of the particles, and hence, permits control over theparticle properties such as hardness. It is believed that particleshaving a high crystallinity, i.e. a large average crystallite size,increase the removal rates of material during CMP processes and thusmake the process more efficient and easier to control. According to oneembodiment of the present invention, the average crystallite size ispreferably at least about 10 nanometers, more preferably is at leastabout 20 nanometers.

The abrasive particles of the present invention also have a high degreeof purity and it is preferred that the particles include no more thanabout 0.1 atomic percent impurities and more preferably no more thanabout 0.01 atomic percent impurities. Impurities in abrasive particlesused for CMP slurries can contaminate the layers in the integratedcircuit, resulting in a defective product. Abrasive particles formed byflame combustion processes often are formed from chloride precursors andinclude contaminants, such as residual chlorides, that are detrimentalto the integrated circuit.

The abrasive particles according to one embodiment of the presentinvention are preferably dense (e.g. not porous), as measured by heliumpycnometry. According to this embodiment, the abrasive particles have aparticle density of at least about 80% of the theoretical value, morepreferably at least about 90% of the theoretical value and even morepreferably at least about 95% of the theoretical value for theparticular abrasive compound. In one embodiment, the particle density isat least about 99% of the theoretical value. High density particlesprovide many advantages over porous particles, including an increasedcompression strength.

However, for some applications, particularly when larger sized particles(e.g. 2-3 μm) are utilized, it may be advantageous to use hollow orporous particles to lower the particle density and enhance the stabilityof the particles in the slurry, particularly when the abrasive compoundis a relatively high density compound. Hollow particles allow theabrasive to remain stable in the slurry for a longer period of time,leading to a more stable slurry and eliminate the need for point-of-usemixing of the abrasive particles and the other slurry components. Themethodology of the present invention advantageously permits control overthe morphology of the particles in this regard. Such larger, hollowparticles can be formed with sufficient compressive strength to be usedfor CMP polishing without crushing and fragmenting.

The abrasive particles of the present invention are also substantiallyspherical. That is, the particles are not jagged or irregular in shape.Spherical abrasive particles are particularly advantageous in CMPslurries because they lead to a more uniform removal rate, reduce thenumber of scratch defects and are easier to clean from the surface ofthe substrate after polishing and planarization due to the reducedsurface area in contact with the substrate. Although the particles aresubstantially spherical, the particles may become faceted and/orequiaxed as the crystallite size increases and becomes close to theaverage particle size. Further, the particle surface may have awrinkled, raisin-like surface structure, although the overall particleis substantially spherical.

In addition, the abrasive powder according to the present invention hasa decreased surface area as compared to prior art powders. Eliminationof larger particles from the powder batches eliminates the porosity thatis associated with open pores on the surface of such larger particles.Due to the elimination of the larger particles and particleagglomerates, the powder advantageously has a lower surface area.Surface area is typically measured using a BET nitrogen adsorptionmethod which is indicative of the surface area of the powder, includingthe surface area of accessible surface pores on the surface of thepowder. For a given particle size distribution, a lower value of asurface area per gram of powder indicates solid and non-porousparticles.

In addition, the powder batches of abrasive particles according to thepresent invention are substantially unagglomerated, that is, theyinclude substantially no agglomerates of particles. Hard agglomeratesare physically coalesced lumps of two or more particles that behave asone larger, non-uniform particle. Agglomerates are disadvantageous inmost CMP applications since they tend to create scratch defects in thesurface of the substrate being polished and also produce erraticpolishing rates. Preferably, no more than about 3 weight percent, morepreferably no more than about 1 weight percent and most preferably nomore than about 0.5 weight percent of the particles are in the form ofhard agglomerates.

According to one embodiment of the present invention, the abrasiveparticles can be composite abrasive particles wherein the individualparticles include an abrasive phase and at least one second phasedispersed throughout the abrasive phase. The second phase canadvantageously be a second abrasive compound. For example, the compositeabrasive particles according to the present invention can includeSiO₂/Al₂O₃, SiO₂/CeO₂, Al₂O₃/CeO₂ or MnO₂/Mn₃O₄. Such compositeparticles can advantageously provide one particle having the desiredmechanical and chemical polishing effects. The ability to incorporate asecond phase in the particles also permits accurate control over theparticle properties, such as hardness, such that one material can beselectively removed in the presence of another during polishing. Also,as is discussed above, the second phase can be a luminescent compoundthat is adapted to enable the detection of residual particles afterpolishing. The ability to incorporate a second phase in the particlesalso permits accurate control over the particle properties, such ashardness, such that one material can be selectively removed in thepresence of another during polishing.

Depending upon the application of the abrasive powder, the compositeparticles preferably include at least about 1 weight percent of thesecond phase, more preferably from about 2 to about 50 weight percent ofthe second phase and even more preferably from about 5 to 35 weightpercent of the second phase.

According to another embodiment of the present invention, the abrasiveparticles can be coated abrasive particles that include a particulatecoating (FIG. 47d) or non-particulate (film) coating (FIG. 47a) on theouter surface of the particles. The coating for abrasive particles is apreferably non-metallic coating, such as a metal oxide or an organiccompound. Preferably, the coating is very thin and has an averagethickness of not greater than about 200 nanometers, more preferably notgreater than about 100 nanometers, and even more preferably not greaterthan about 50 nanometers. While the coating is thin, the particulate ornon-particulate coating should substantially encapsulate the entireparticle. Accordingly, the coating preferably has a thickness of atleast about 5 nanometers. The coating can be, for example, silica,alumina or ceria. In one embodiment of the present invention, theabrasive particle comprises SiO₂ with an Al₂O₃ coating encapsulating theSiO₂ core or SiO₂ particles with a CeO₂ coating. Such coatings can beproduced in a manner discussed hereinabove. Such coatings canadvantageously provide accurate control over the particle density, forexample when a high density coating is placed on a low density particle,and can also provide control over the chemical and mechanical action ofthe particles during polishing.

The abrasive particles can also include a coating of an organic compoundsuch as PMMA (polymethylmethacrylate), polystyrene or the like. Theorganic coating preferably has a thickness of not greater than about 50nanometers and is substantially dense and continuous about the particle.The organic coatings can advantageously prevent corrosion of theparticles and also can improve the dispersion characteristics of theparticles in the slurry. For example, a coating can influence the redoxcharacteristics of the particles. Such coatings can also extend theshelf-life of the slurries. Presently available slurries have a limitedshelf-life primarily due to the high surface area of the particles,which exposes the surfaces to the slurry components, causing thesurfaces to become hydrophobic.

The coating can also be comprised of one or more monolayer coatings,such as from about 1 to 3 monolayer coatings. A monolayer coating isformed by the reaction of an organic or an inorganic molecule with thesurface of the particles to form a coating layer that is essentially onemolecular layer thick. In particular, the formation of a monolayercoating by reaction of the surface of the abrasive particle with afunctionalized organo silane such as halo- or amino-silanes, for examplehexamethyldisilazane or trimethylsilylchloride, can be used to modifythe hydrophobicity and hydrophilicity of the powders. Such coatings canpermit greater control over the dispersion characteristics of the powderin a slurry.

The monolayer coatings may also be applied to abrasive powders that havealready been coated with an organic or inorganic coating thus providingbetter control over the corrosion characteristics as well asdispersibility of the particles.

In addition to the foregoing, the coating can be a consumable coatingwherein the coating is chemically active and is consumed duringpolishing such that the polishing action substantially stops when thecoating is consumed. Such a coated abrasive particle could be used toremove a pre-selected amount or thickness of material, effectivelybehaving as an etch-stop mechanism.

The abrasive powders according to the present invention, includingcomposite powders and coated powders, are useful in a number ofapplications and can be used to fabricate a number of intermediateproducts, such as CMP slurries and other polishing slurries or pastes.Such intermediate products are included within the scope of the presentinvention.

As is discussed above, the abrasive powders of the present invention canbe used in CMP slurry compositions. A typical CMP slurry consists ofabrasive particles, which are usually prepared by gas combustiontechniques (fumed) or are prepared by solution precipitation methods.The concentration of the abrasive particles in a CMP slurry is typicallyfrom about 0.5 to 60 weight percent, preferably from about 1 to about 20weight percent, such as from about 7 to 15 weight percent. Generally,higher concentrations of abrasive particles will lead to higher polishrates.

Water, such as deionized or distilled water, is typically a majoringredient in CMP slurries and is the liquid in which the abrasiveparticles are dispersed and other components are dissolved. The pH ofthe aqueous based slurry is typically adjusted with acids or bases,often utilizing a buffer system such as acetic acid/sodium acetate whichallows the slurry to resist changes in the pH. The pH can be adjustedaway from the isoelectric point of the abrasive compound to help aid inparticle suspension.

Another class of chemical ingredient typically added to CMP slurries aresurfactants which also aid in the stability of the slurry by helping toprevent particle agglomeration, flocculation and settling. Dispersiveagents such as these are typically incorporated in the slurry in anamount of up to about 20 weight percent. Agglomeration, precipitation,flocculation and settling of the abrasive particles will drasticallyeffect the performance and shelf life of the slurry. Surfactants caninclude the alkali sulfonates, sulfates, lignosulfonates, carboxylatesand phosphates.

Another class of chemical ingredient typically added to metal CMPslurries are oxidizing agents. The oxidizing agents are added to theslurry to oxidize the metal surface to the metal oxide which is thenmechanically abraded. Examples of oxidizing agents include, but are notlimited to, oxidizing metal salts, chlorates, perchlorates, chlorites,iodates, nitrates, persulfates, peroxides, ozinated water and oxygenatedwater. Oxidizing agents are typically incorporated in the slurry atconcentrations of up to about 20 weight percent.

Further, the CMP slurries, particularly metal CMP slurries, can includecomplexing agents. Complexing agents, such as ammonia, can be beneficialin aiding in the dissolution of the removed metal species into theslurry.

CMP slurries are primarily utilized to polish and planarize differentlayers during the production of integrated circuits. Typically, fromabout 3 to 10 CMP steps are required to manufacture an integratedcircuit. The surface being treated can include a metal oxide, such as aninterlayer dielectric, a metal film and/or an organic material, such asa polymer film. Common metal films and plugs which are planarized duringintegrated circuit manufacture include tungsten, copper, aluminum andtantalum. Common metal oxides which are planarized during integratedcircuit manufacture include silica, interlayer dielectrics, “low K”dielectric materials and other insulative materials.

Depending upon the surface being polished, the slurry can includeabrasive particles of one or more of the abrasive compounds disclosedherein. For many applications, it will be desirable to include at leasttwo different compounds, such as SiO₂ and CeO₂, to achieve a particularcombination of mechanical and chemical action. Generally, the slurry isapplied to a pad which is then rotated and placed against the integratedcircuit surface.

A device for planarizing an integrated circuit is illustrated in FIG.50. The device includes a polishing pad 390 (or platen) which is adaptedto be rotated upon its axis. The polishing pad 390 is typicallyfabricated from a material such as polyurethane. The integrated circuitwafer 392 is brought into contact with the polishing pad 390 withapplied pressure P, typically from about 3 to 7 psi. In the embodimentillustrated in FIG. 50, both polishing pad 390 and the wafer 392 areslowly rotated simultaneously, typically in the same direction.

A CMP slurry is housed in a reservoir 394. The reservoir 394 providesslurry via slurry conduits 396 to the polishing pad. Slurry is providedto the polishing pad 390 in a controlled fashion so that the polishingrate of the wafer 392 is controlled.

FIG. 51 illustrates the various stages in the formation of theintegrated circuit layer, including a chemical mechanical planarizationstep, wherein the integrated circuit layer includes a metal plug, suchas a tungsten plug. The integrated circuit layer includes a siliconsubstrate 402 including channels 404 which are coated with a dielectricmaterial 406. A coating process is used to deposit a further metal layer408 over the dielectric layer 406. The metal layer 408 is nonuniform andproduces a non-planarized surface.

During the chemical mechanical planarization step, a CMP slurry 410 isplaced between the metal layer 408 and a polishing pad 412. As thepolishing pad 412 rotates, it abrades the slurry 410, including theabrasive particles, against the metal layer 408 until the metal layer408 and the upper surface of the dielectric layer 406 are removed fromthe substrate creating a metal plug 408.

A damascene process is illustrated by FIG. 52. In the damascene process,an insulating material is deposited on the wafer, which is thenpatterned and etched to form voids. A metal, such as copper, is thendeposited from a chemical vapor to fill the voids and cap the wafer. Theexcess metal is then removed to create a smooth surface inlaid withconductive vias and traces. Referring to FIG. 52, a wafer 420 includes asilicon layer 422 overlaid with a dielectric material 424. Thedielectric material 424 is then pattern etched to form voids 426.Thereafter, a metal layer 428 is deposited over the dielectric layer424. During the chemical mechanical planarization step, a CMP slurry 430is placed between the metal layer 428 and a polishing pad 432 and thepolishing pad 432 rotates to abrade the slurry 430, including theabrasive particles, against the metal layer 428 until the metal layer428 Is removed from the substrate. The resulting structure consists ofinterconnect metal 434 and interlayer dielectric 436.

The CMP slurries of the present invention provide numerous advantagesduring this process. The use of the slurries comprising abrasiveparticles having the properties described herein will result in highlyreliable polishing operations. Number of scratch defects, the RMSroughness and surface contamination will be significantly reduced.Further, the abrasive particles of the present invention will permit theuse of a lower pressure on the polishing pad against the integratedcircuit surface.

The foregoing advantages of the present invention can be understood withreference to FIGS. 53 and 54. FIG. 53 illustrates the polishing of asurface feature with a conventional CMP slurry. The CMP slurry includesparticles 420 which are abraded against a surface feature 422 under anapplied pressure P. The particles 420 are agglomerated andnon-spherical, and result in a number of scratches on the surface 422 ofthe feature. Scratches and other surface defects produced by suchparticles directly result in a defective product.

A similar process utilizing the slurry according to the presentinvention is illustrated in FIG. 54. The particles 424 are spherical anddo not include any sharp or jagged edges. Therefore, when the particles424 are abraded against the surface 426 the resulting surface 426 has aminimal number of scratches or other defects that can produce adefective integrated circuit product.

After planarizing of the integrated circuit layer using the CMP slurry,the slurry must be cleaned from the surface before further processing ofthe integrated circuit. Small agglomerated particles are difficult tocompletely remove from the surface of the integrated circuit. Sphericalparticles are easier to remove since such particles flow more easilyalong the surface of the integrated circuit layer. For someapplications, the abrasive particles are embedded into the polishing padrather than placed into a slurry.

EXAMPLES

Cerium oxide (CeO₂) is a preferred abrasive compound according to anembodiment of the present invention. Accordingly, the effect ofdifferent process variables on the production of cerium oxide particleswas evaluated. For each of the following examples, precursor solutionswere atomized using an ultrasonic transducer. For each example, animpactor was used to narrow the droplet size distribution in the aerosolby removing droplets from the aerosol. The cut-point for the impactorwas about 10 μm. Air was used as the gas to carry the aerosol throughthe heated reaction zone. The precursor concentrations are given as aweight percent of the oxide in solution.

A. Effect of Precursor Type

Examples 1-3, summarized in Table I, illustrate the effect of the typeof precursor on the formation of the abrasive particles.

TABLE I Effect of Precursor Type Precursor Reactor Precursor Concen-Other Transducer Tem- Example Type tration Additives Frequency perature1 CAN 5 w/o — 1.6 MHz 1000° C. 2 CN 5 w/o — 1.6 MHz 1000° C. 3 CC 5 w/o— 1.6 MHz 1000° C.

Referring to Table I, CAN stands for cerium (IV) ammonium nitrate whichhas a chemical formula of (NH₄)₂Ce(NO)₆, CN stands for cerium nitratehexahydrate which has a chemical formula of Ce(NO)₃.6H₂O and CC standsfor cerium chloride which has a chemical composition of CeCl₃.7H₂O. Eachof these precursors was formed into a solution including 5 weightpercent of the oxide and an aerosol was created from the solution usingan ultrasonic transducer. The aerosol was carried through a tube furnaceheated to 1000° C.

The particles produced from the CAN precursor are illustrated in FIG.55. The particles have a wrinkled raisin-like surface on hollowparticles. FIG. 56 illustrates the particles produced from the CNprecursor. The particles produced from the CN precursor had a smoothersurface, however, there are many rigid walled hollow particles. Thismorphology is undesirable for CMP applications because of the likelihoodfor disintegration of the particles during use resulting in uncontrolledpolishing. FIG. 57 illustrates the particles produced from the CCprecursor. The particles had a desirable spherical morphology. Althoughthere is still some retention of hollow particles in the powder batch,the number of hollow particles is reduced as compared to Example 2.

In summary, the CAN and CC are the preferable precursors since thenumber of hollow particles is minimal. The CAN precursor is morepreferable primarily because decomposition of the CC forms HCl, whichcan cause significant corrosion of the process equipment.

B. Effect of Precursor Concentration

Examples 1 and 4 (Table II) illustrate the effect of precursorconcentration on the properties of the final particles.

TABLE II Effect of Precursor Concentration Precursor Reactor PrecursorConcen- Other Transducer Tem- Example Type tration Additives Frequencyperature 1 CAN 5 w/o — 1.6 MHZ 1000° C. 4 CAN 1 w/o — 1.6 MHZ 1000° C.

The powder produced according to Example 1 is illustrated in FIG. 55.The average particle size is about 1 μm and the particles have awrinkled surface. FIGS. 58 and 59 illustrate the powder producedaccording to Example 4, from a 1 weight percent precursor solution. Theparticle size decreased to about 0.7 μm and the particles had aspherical morphology with smooth outer surfaces. Very few of theparticles appeared to be hollow. Thus, decreasing the concentration ofthe precursor solution reduces the average particle size of the abrasiveparticles, and the particles have a substantially spherical morphology.The powder also appears to have a narrower size distribution. Such apowder is well-suited for use in CMP slurries according to the presentinvention.

C. Frequency of Atomization

The preferred technique for generating an aerosol according to thepresent invention is by the use of an ultrasonic transducer. The effectof the transducer frequency on the properties of abrasive particles wasinvestigated, as summarized in Table III.

TABLE III Effect of Atomization Frequency Precursor Precursor TransducerReactor Example Type Concentration Frequency Temperature 1 CAN 5 w/o 1.6MHZ 1000° C. 5 CAN 5 w/o 2.4 MHZ 1000° C.

The powder produced according to Example 1 is illustrated in FIG. 55 andthe powder produced according to Example 5 is illustrated in FIG. 60.When the frequency was increased from 1.6 MHZ to 2.4 MHZ, the averagesize of the abrasive particles decreased from about 1 μm to about 0.7μm. Although the average particle size decreased, the particles retaineda wrinkled morphology. This result suggests that the surface morphologyof the particles is primarily controlled by the precursor type andconcentration.

D. Effect of Reactor Temperature

An experiment was conducted to determine the effect of varying thereactor temperature on the formation of the abrasive particles. Theresults are illustrated by Examples 6 and 7 (Table IV).

TABLE IV Effect of Reactor Temperature Precursor Precursor TransducerReactor Example Type Concentration Frequency Temperature 6 CAN 3 w/o 2.4MHz 1000° C. 7 CAN 3 w/o 2.4 MHz  600° C.

FIG. 61 (Example 6) and FIG. 62 (Example 7) illustrate the effect thatthe reactor temperature has on the morphology and crystallinity of theabrasive particles. At 600° C., the particles had a similar averageparticle size as the particles produced at 1000° C., but had a smootherand more spherical morphology. Lowering the reaction temperaturedecreases the amount of surface wrinkles and produces smoother and morespherical particles. However, some particles were hollow, as evidencedby presence of fragments. Also, the relative average crystallite sizedecreased from about 25 nanometers to about 11 nanometers when thetemperature was decreased from 1000° C. to 600° C.

E. Effect of Additives to Precursor Solution

The effect of different additives to the precursor solution was alsoinvestigated. These Examples are illustrated in Table V.

TABLE V Effect of Additives to the Precursor Solution Precursor Trans-Reactor Ex- Precursor Concen- Other ducer Tem- ample Type trationAdditives Frequency perature  1 CAN 5 w/o   — 1.6 MHz 1000° C.  8 CAN 5w/o 1 eq. urea 1.6 MHz 1000° C.  2 CN 5 w/o   — 1.6 MHz 1000° C.  9 CN 5w/o 1 eq. urea 1.6 MHz 1000° C. 10 CAN 3 w/o 3 eq. glucose + 2.4 MHz1000° C. 3 eq. urea  6 CAN 3 w/o   — 2.4 MHz 1000° C. 11 CAN 3 w/o 3 eq.glucose 2.4 MHz 1000° C. 12 CAN 3 w/o 3 eq. urea 2.4 MHz 1000° C.

Urea and glucose were added to the precursor solution in an attempt tofurther densify the particles. It is believed that the additives can aidin the densification of the particles by at least two mechanisms. First,the additives may act as a fuel which causes the particles to burst orexplode upon ignition creating debris and fragments which can furthersinter into smaller particles with increased density. Ideally, the fuelwould cause the dry particle to explode before densification or causethe initial droplets to explode into smaller droplets. In the secondmechanism, the additive acts as a chemical binder to link the precursormolecules together, diminishing the formation of hollow particles.

FIGS. 63 and 64 illustrate Examples 8 and 9, which demonstrate theeffect of the addition of urea to CAN and CN precursor solutions. Theeffect on the particles made from CAN (FIG. 63) were minimal. Theparticles have a wrinkled surface and the size decreased only slightly.The effect was more pronounced for the particles produced from CN,illustrated in FIG. 64. The addition of urea to the precursor resultedin particles having a reduced particle size and fewer particles appearedto be hollow, as compared to Example 2 (FIG. 56).

Examples 10, 6, 11 and 12 illustrate the effect of higher concentrationsof the additives urea and glucose when the precursor solution isatomized at 2.4 MHz. FIG. 59 (Example 6) illustrates the powder withoutany additives, which has a wrinkled surface morphology. The addition ofurea (Example 12, FIG. 65) does not alter the surface morphology, butthe average particle size appears to be slightly smaller. The additionof glucose (Example 11, FIG. 66) produced many fragments and debris. Theaddition of both urea and glucose together, Example 10 (FIG. 67),produced large spherical particles with a high degree of hollowness.Thus, glucose does not appear to be an effective additive and the use ofboth additives together appears to be counter productive.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

What is claimed is:
 1. A chemical mechanical planarization slurry,comprising a liquid carrier and from about 0.5 to about 60 weightpercent of abrasive particles comprising a first abrasive compoundselected from the group consisting of Al₂O₃, CeO₂ and ZrO₂ dispersedthroughout said liquid carrier, wherein said abrasive particles aresubstantially spherical and have a weight average particle size of fromabout 0.1 to 3 μm and a particle size distribution wherein at leastabout 90 weight percent of said particles are not larger than twice saidaverage particle size, wherein said abrasive particles are compositeabrasive particles comprising said first abrasive compound and a secondabrasive compound.
 2. A slurry as recited in claim 1, wherein saidabrasive particles are Al₂O₃.
 3. A slurry as recited in claim 1, whereinsaid abrasive particles are CeO₂.
 4. A slurry as recited in claim 1,wherein said abrasive particles are ZrO₂.
 5. A slurry as recited inclaim 1, wherein said slurry comprises first abrasive particles of afirst abrasive compound and second abrasive particles of a secondabrasive compound.
 6. A slurry as recited in claim 1, wherein at leastabout 95 weight percent of said abrasive particles are not larger thantwice said average particle size.
 7. A slurry as recited in claim 1,wherein said weight average particle size is not greater than about 1μm.
 8. A slurry as recited in claim 1, wherein said weight averageparticle size is from about 0.1 μm to about 0.75 μm.
 9. A slurry asrecited in claim 1, wherein said abrasive particles have an averagecrystallite size of at least about 20 nanometers.
 10. A slurry asrecited in claim 1, wherein not greater than about 1 weight percent ofsaid abrasive particles are in the form of hard agglomerates.
 11. Aslurry as recited in claim 1, wherein said abrasive particles have aparticle density of at least about 90 percent of the theoretical densityof said abrasive compound.
 12. A slurry as recited in claim 1, whereinsaid abrasive particles are hollow shells.
 13. A slurry as recited inclaim 1, wherein said liquid carrier is an aqueous-based slurry.
 14. Aslurry as recited in claim 1, wherein said slurry comprises from about 1to about 20 weight percent of said abrasive particles.
 15. A slurry asrecited in claim 1, wherein said liquid carrier is an aqueous-basedslurry comprising an oxidizing agent.
 16. A slurry as recited in claim1, wherein said liquid carrier is an aqueous-based slurry comprising acomplexing agent.
 17. A slurry as recited in claim 1, wherein saidabrasive particles are coated particles.
 18. A slurry as recited inclaim 1, wherein said abrasive particles are coated particles comprisinga coating adapted to be consumed during use of said slurry in apolishing process.
 19. A slurry as recited in claim 1, wherein saidabrasive particles are coated particles comprising a metal oxidecoating.
 20. A slurry as recited in claim 1, wherein said abrasiveparticles are coated particles comprising an organic coating.
 21. Aslurry as recited in claim 1, wherein said abrasive particles arecomposite abrasive particles comprising a first abrasive compound and asecond phase.
 22. A slurry as recited in claim 1, wherein said abrasiveparticles are composite abrasive particles comprising a first abrasivecompound and a luminescent compound.
 23. A chemical mechanicalplanarization slurry, comprising a liquid carrier and abrasive particlesdispersed throughout said liquid carrier, wherein said abrasiveparticles have a weight average particle size of from about 0.1 μm toabout 3 μm and wherein said abrasive particles are coated abrasiveparticles, wherein said slurry comprises first coated abrasive particlesof a first abrasive compound and second coated abrasive particles of asecond abrasive compound.
 24. A chemical mechanical planarizationslurry, comprising a liquid carrier and abrasive particles dispersedthroughout said liquid carrier, wherein said abrasive particles have aweight average particle size of from about 0.1 μm to about 3 μm andwherein said abrasive particles are coated abrasive particles, whereinsaid coated abrasive particles comprise a metal oxide coating.
 25. Achemical mechanical planarization slurry, comprising a liquid carrierand abrasive particles dispersed throughout said liquid carrier, whereinsaid abrasive particles have a weight average particle size of fromabout 0.1 μm to about 3 μm and wherein said abrasive particles arecoated abrasive particles, wherein said coated abrasive particlescomprise a CeO₂ coating.
 26. A chemical mechanical planarization slurry,comprising a liquid carrier and abrasive particles dispersed throughoutsaid liquid carrier, wherein said abrasive particles have a weightaverage particle size of not greater than about 3 μm and wherein saidabrasive particles are composite abrasive particles comprising a firstabrasive compound and a second phase, wherein said second phase is asecond abrasive compound.
 27. A chemical mechanical planarizationslurry, comprising a liquid carrier and abrasive particles dispersedthroughout said liquid carrier, wherein said abrasive particles have aweight average particle size of not greater than about 3 μm and whereinsaid abrasive particles are composite abrasive particles comprising afirst abrasive compound and a second phase, wherein said compositeabrasive particles comprise a first abrasive compound selected from thegroup consisting of Al₂O₃ and SiO₂ and said second phase is CeO₂.
 28. Achemical mechanical planarization slurry, comprising a liquid carrierand abrasive particles dispersed throughout said liquid carrier, whereinsaid abrasive particles are substantially spherical hollow particleshaving a weight average particle size of from about 0.1 μm to about 3μm.
 29. A slurry as recited in claim 28, wherein said average particlesize is not greater than about 2 μm.
 30. A slurry as recited in claim28, wherein said abrasive particles comprise a metal oxide.
 31. A slurryas recited in claim 28, wherein said abrasive particles comprise acompound selected from the group consisting of Al₂O₃, SiO₂, CeO₂ andZrO₂.
 32. A slurry as recited in claim 28, wherein not greater thanabout 3 weight percent of said particles are in the form of hardagglomerates.
 33. A slurry as recited in claim 28, wherein said slurrycomprises from about 0.5 to about 60 weight percent of said particles.34. A chemical mechanical planarization slurry, comprising a liquidcarrier and abrasive particles dispersed throughout said liquid carrier,wherein said abrasive particles comprise a phosphor compound and have aweight average particle size of not greater than about 3 μm.
 35. Aslurry as recited in claim 34, wherein said average particle size is notgreater than about 2 μm.
 36. A slurry as recited in claim 34, whereinsaid average particle size is from about 0.1 μm to about 0.75 μm.
 37. Aslurry as recited in claim 34, wherein said abrasive particles arecomposite abrasive particles comprising an abrasive compound and aluminescent compound.
 38. A slurry as recited in claim 34, wherein saidslurry comprises from about 0.5 to about 60 weight percent of saidabrasive particles.
 39. A chemical mechanical planarization slurry,comprising a liquid carrier and abrasive particles dispersed throughoutsaid liquid carrier, wherein said abrasive particles have a weightaverage particle size of not greater than about 3 μm and wherein saidabrasive particles are composite abrasive particles comprising a firstabrasive compound and a luminescent compound.
 40. A slurry as recited inclaim 39, wherein said composite abrasive particles are substantiallyspherical.
 41. A slurry as recited in claim 39, wherein said firstabrasive compound is selected from the group consisting of Al₂O₃, SiO₂,CeO₂, ZrO₂, and mixtures thereof.
 42. A slurry as recited in claim 39,wherein said weight average particle size is not greater than about 2μm.